Nonlinear Nano-Optics: Optical Nonlinearities of Gold Nano-Structures at the Transition from Classical to Quantum Coupling

Understanding optical nonlinearities in nano-structures is key for novel nanospectroscopy methods and optical information processing. For Au nanostructures, the optical response from intraband electronic transitions give rise to efficient third-order nonlinear emissions, including the coherent four-wave mixing (FWM) and incoherent hot electron (HE) response. However, there is debate regarding the nano-structure incoherent emission mechanism and how it is modified by quantum effects at small interparticle separations. Emission at the separation depends on both optical quenching from near-field coupling and classical enhancement from changes in electromagnetic densities. Here, we use adiabatic nanofocusing of surface-plasmon polaritons on a Au nano-tip to investigate the FWM and HE response in the near-IR range at the transition from long-range classical resonant energy transfer to quantum coupling. We combine atomic force and scanning tunneling distance control to approach a Au surface with subnanometer precision and explore the unique optical behavior at the tunneling regime to better understand the debated emission mechanisms. We demonstrate that the classical field-enhanced behavior dominates until quantum coupling dramatically reduces emission intensity and field enhancement.